1. Field of the Invention
The present invention relates to a medical device for detecting hemodynamic conditions of a heart, in particular conditions corresponding to dangerous arrhythmias.
2. Description of the Prior Art
An electromedical implant for electrical treatment of a heart which may exhibit dangerous arrhythmias must be capable of identifying the presence of these conditions. Detection of, e.g., ventricular fibrillation (VF), however, is associated with difficulties in the prior art. One such difficulty is that the detection of VF from, e.g., the ECG signal (the heart's depolarization signal) requires knowledge of the ECG signal's morphology, as illustrated in U.S. Pat. No. 5,058,599.
This patent discloses a method and a device for detecting a dangerous heart condition wherein a sequence of abnormal events is detected among a plurality of normal events in an electric signal, especially in a depolarization signal in the heart. According to this known method, a plurality of signal parameters, typical of the event to be detected, are identified. One such parameter can be, e.g., the signal's derivative within a special tracing segment. After normal and abnormal events are extracted from the signal, a statistical calculation, based on all recorded events for a sequence of the identified signal parameters, is performed. or example, standard deviation is calculated, and this value is compared to a predefined threshold value. If the value for the standard deviation exceeds the threshold value, this indicates the presence of an abnormal condition.
In order to come to grips with this difficulty and to achieve an ability to select reliable triggering points in an individual signal tracing in order to identify, e.g., the tracing slope during a specific interval, use of the ECG signal according to the procedure shown in U.S. Pat. No. 4,453,551 could be a solution.
A device and a method for analyzing and distinguishing dangerous fibrillation conditions from the heart's normal sinus rhythm as well as from other conditions in the heart, such as ventricular tachycardia, are also known from U.S. Pat. No. 4,453,551. This is achieved by the study of a number of tests performed on a extracorporeally recorded ECG signal. Before these tests can be performed, the signal is sampled, filtered and standardized. In standardization, the amplitude of the signal, which varies greatly from one patient to another, is standardized (normalized). According to one of these tests, the ratio between the energy in an ECG signal, above and below a zero level, is calculated in a time window. In ventricular tachycardia or sinus rhythm which, in contrast to fibrillation, displays a regular ECG signal, the calculated energy ratios are largely constant in a series of consecutive time windows, making it possible to distinguish tachycardia from fibrillation. An alternative calculation method for this test is to calculate and study variations in the energy ratios for a series of consecutive time windows.
Another device in which the ECG signal is utilized for detecting ventricular fibrillation and ventricular tachycardia is disclosed in U.S. Pat. No. 4,870,974. In this known device, the heart's hemodynamic state is studied with the aid of pressure signals, in addition to a study of the ECG signal's tracing morphology. For the pressure signal, the time that the measured pressure is below a specific level is determined, with different threshold values are for e.g. fibrillation and tachycardia being used in this determination. As with the device according to U.S. Pat. No. 5,058,599, certain parameters must also be identified in U.S. Pat. No. 4,870,974 from an individual signal tracing. For the pressure signal, a reference level, for example, must be identified from the signal tracing for detection to work.